Contemporary portable electronic appliances rely almost exclusively on rechargeable Li-ion batteries as the source of power. This has spurred a continuing effort to increase their energy storage capability, power capabilities, cycle life and safety characteristics, and decrease their cost. A lithium-ion battery or lithium ion cell refers to a rechargeable battery having an anode capable of storing a substantial amount of lithium at a lithium chemical potential above that of lithium metal.
By way of example, consider a battery pack that is formed from a number of lithium cells connected together in series. The lifetime of the lithium battery pack degrades if the voltage across one of its cells falls below a predetermined threshold during discharge (e.g. 1.5 volts), or rises above a predetermined threshold during charging (e.g., 3.9 volts). For this reason the prior art has traditionally monitored carefully cell voltages and taken measures to maintain cell voltages in a particular range.
To complicate matters further, manufacturing defects in lithium cells result in some cells that do not hold as much charge as other seemingly identical cells. For this reason, when a number of lithium cells are connected together in series, defective cells discharge more quickly than the other cells and more quickly reach the lower threshold described above during discharge. Similarly, such defective cells are often the first to hit the upper threshold during charging. This imbalance between cells limits the effective range of operation of the battery pack, unless the charge is rebalanced during operation.
Inside a lithium ion battery, there are a number of ions which migrate throughout the cell as the cell's state of charge is changed. For example, the ions are stored in a specific location of a lattice structure in a full SOC condition, as is known in the art. As the SOC depletes or the cell discharges energy, the ions within the cell move to a different lattice structure in a different location of the cell. The flow of electrons is caused by a load that enables the movement of ions within the cell. In a series configuration of lithium ion cells, e.g., a battery pack, the migration of ions occurs in each individual cell. If an individual cell within a battery pack were to deplete, i.e., have no more ions to move over to the discharge lattice structure, voltage will build on that cell causing potentially irreparable harm to the cell or battery pack.
Lithium ion cells, as opposed to nickel metal hydride or nickel cadmium cells, are not as naturally balanced. Accordingly, management of SOC of a battery pack including lithium ion cells has traditionally required an accounting for the SOC in each individual cell. Prior art systems include balancing mechanisms to make sure each cell has a similar amount of ions. As an alternative, it has been attempted to meticulously manufacture batteries with identical cells such that each cell reaches an SOC and SOD at the same time. The prior art approaches, however, are disadvantageous as a manufacturer has to incur additional costs to ensure precise quality control and monitoring of each individual cell.
When lithium ion batteries or capacitors or other electrochemical generators such as hybrid or “asymmetric” devices comprising both capacitive and faradic storage are joined in a series string in order to obtain a higher voltage than a single cell can provide, repeated charge and discharge of the string can result in cells getting “out of balance” such that the state-of-charge of cells varies along the string. The out-of-balance condition can arise from differences in the rate of capacity fade upon cycling of the cells, or variations in impedance leading to differences in capacity fade rate, amongst other causes. The string may not be perfectly balanced to begin with, with the cells in the series string varying in their capacity or state-of-charge upon assembly of the string. There are many applications of such packs, including, but not limited to, power tool or appliance batteries, electric vehicle batteries, and batteries for backup power.
An out-of-balance series string such as described here can be undesirable for several reasons. Upon charging of the series string, a cell of lower capacity or higher SOC can be overcharged when the string is charged to a specified pack voltage, resulting in premature failure of the pack or an unsafe condition, such as venting of the cell or thermal runaway or explosion. The same can occur upon discharge of an imbalanced string. In extreme cases, discharge of an out-of-balance pack can send certain cells into “voltage reversal” where the polarity of a cell is opposite to that during normal use. In a lithium ion cell this can result in dissolution of the negative current collector causing failure or gas generation causing mechanical rupture of the cell. The prior art approach to avoiding the problems associated with an imbalanced string is to have individual cell monitoring and balancing, which requires additional control circuitry and increases the cost and complexity of the battery configuration.